Abstract Cold and Dense Samples of Naphthalene (C10H8) Are Produced

Abstract Cold and dense samples of naphthalene (C10H8) are produced using bu®er gas cooling in combination with rapid, high flow molecule injection. The observed naphthalene density is n ¼ 1011 cm¡3 over a volume of a few cm3 at a temperature of 6 K. We observe naphthalene-naphthalene collisions through two-body loss of naphthalene with a loss cross section ¡14 2 of σN ¡N = 1:4£10 cm . Analysis is presented that indicates that this combination of techniques will be applicable to many comparably sized molecules. This technique can also be combined with cryogenic beam methods 1 to produce cold, high flux, continuous molecular beams. 1 Cooling and Collisions of Large Gas Phase Molecules David Patterson, Edem Tsikata, and John M. Doyle February 5, 2010 1 Introduction Driven by a variety of new science, including cold chemistry and dipolar quan- tum gases, several methods are now being pursued to produce cold and ultracold samples of molecules. The cold molecules in these studies are generally diatomic 2 but also include few-atom molecules such as ND3. Extending this work to the cooling of larger molecules is of high interest, as reviewed by Meijer et. al. 3 and references therein. For example, chemical reaction rates at low temperatures are of great current interest, and extending these studies to important large molecules is essential. Similarly, ultraprecise spectroscopy applications could make use of new continuous cryogenic molecular beams. Finally, there is great interest in ¯eld mediated chemistry, and a general source of high density, highly polarizable ground state molecules is an excellent testbed for observing ¯eld mediated chemical reactions. In previous work with bu®er gas cooling, cold, guided beams of molecules 4 1 as large as trifluoromethane (CF3H) have been produced. In related work, a beam of slow (11 m s¡1), but rotationally and vibrationally warm (300 K), PerfluoroC60 (mass > 6000 amu) was produced by ¯ltering slow molecules from a warm sample. 5 However, the only demonstrated technique for producing samples of cold molecules with atom number higher than ¯ve is the seeded supersonic jet. Seeded supersonic beams have a rich history and have found great utility in spectroscopic studies, 6 as sources for molecular trapping experiments, 3 and in cold chemistry experiments using the CRESU technique. 7 They are lim- ited, however, because although they produce translationally and rotationally cold molecules, these molecules are moving at very high velocity (300 m s¡1 or higher). Furthermore, the beam evolves spatially with a rapidly decreasing density as the molecules move away from the beam ori¯ce. New cold beam methods (some mentioned just above) are part of a renaissance in molecular beams. Part of this renaissance is the use of electric and magnetic ¯elds to manipulate and decelerate polar and magnetic molecules. In many cases, these methods are applied to molecules in metastable states, such as low-¯eld seeking electric dipole states. Extending these slowing and trapping methods to larger molecules is inherently problematic because, unlike diatomics 2 and few-atom molecules, polyatomic molecules e®ectively have only high ¯eld seeking states due to their small rotational splittings. This nearly eliminates low-¯eld seeking electric guides and decelerators from applicability. To overcome these constraints, ingenious techniques have been ¯elded to align 8, 9 separate, 10 and decelerate 3 larger molecules. Great progress has been made manipulating these molecules, but producing samples of very large, cold molecules at rest in the laboratory frame has remained elusive. Here we report on the creation of gas-phase naphthalene at 6 K, created through a novel rapid helium gas cooling method. Our ¯ndings answer an important question about cluster formation in these cold bu®er-gas systems. Speci¯cally, prior to this work, it was an open question whether larger molecules in a cryogenic helium gas would rapidly accumulate a layer of bound helium atoms or instead remain \naked". In the latter case the naphthalene would cool to the helium temperature but remaining free of adsorbed helium atoms. It is easy to see why clustering could be expected { the high binding energy of a helium atom to the molecule, in combination with the rich vibrational mode structure of larger molecules and the low temperature of the helium gas, could lead to the creation of long lived excited molecule-helium dimers, which would initiate clustering. The creation of 6 K naphthalene described in this work demonstrates that this is not the case, and thus greatly increases the potential scope of bu®er gas cooling as a production method for cold, gas phase molecules. To our knowledge, this work represents the ¯rst observation of cold (< 10 K), larger (> 5 atoms) molecules that are moving slowly in the laboratory rest frame. As we will describe in detail, in addition to cooling naphthalene and placing limits on the He-naphthalene clustering rate, we were also able to observe the loss of naked naphthalene due to cluster initiation from two body naphthalene-naphthalene collisions. Our studies o®er evidence that rapid clustering in a cryogenic gas depends critically on the vibrational properties of the cluster constituents. A simple model suggests that our cooling methodology will be applicable to a wide variety of molecules of size comparable to (or smaller than) naphthalene. 1.1 Apparatus A cold mixture of naphthalene (C10H8, denoted N in this paper) and helium is produced by injecting a hot (300 K) mixture into a cold cell tube via a short (1 cm), thermally isolating transition tube. Our apparatus, shown in ¯gure 1, is a qualitative change from the cooling system described in. 1 In that work, cold beams of potassium and ammonia were produced by cooling entrained mixtures with neon bu®er gas and flowing the mixture through an aperture into vacuum. In this work, the beam aperture was replaced by a pumping line. The lower oven temperature and design improvements allow us to use a signi¯cantly shorter transition tube in comparison with earlier work, reducing input losses and allowing us to run with substantially lower bu®er gas flows, leading to lower in-cell helium densities. These crucial changes allowed for the success of these experiments. 3 300 K 77 K vacuum radiation chamber shield 6 K cell tube Ultem transition tube quartz windows Figure 1: A mixture of helium bu®er gas and naphthalene flows down a tube from 300 K, where the naphthalene has signi¯cant vapor pressure, to a cryogenic cell anchored to a liquid helium bath. Considerable care is taken to keep the transition region as short as possible while maintaining an adequate thermal disconnect. This is achieved by an Ultemr tube, length 10 mm, wall thickness 0.5 mm, between the 300 K input tube and the cold cell tube. The cell tube is thermally anchored to the helium bath of a small cryostat; it is then connnected via a second Ultemr tube to a 2 cm diameter pumping line. The total heat load on the helium bath with no bu®er gas flowing is ¼ 500 mW, probably dominated by blackbody from the 300 K pumpout line; with a typical flow of 100 sccm of helium, this heat load increases to ¼ 900 mW. The helium pressure is 2 Torr at the tube input and 150 mTorr at the tube output. The mixture begins to cool as soon as it enters the cold cell tube. Any molecule which di®uses to a cold wall sticks and is lost from the gas. A species A entrained in the bu®er gas will cool with little loss as long as the elastic scattering cross section σA¡He > σHe¡He. Although the low temperature cross section σN¡He is not known, the success of this experiment indicates that this criteria appears to be met for naphthalene, in agreement with the basic expectation that a naphthalene molecule is physically larger than a helium atom. As the mixture enters the cell and thermalizes, the phase space density of naphthalene increases by more than 5 orders of magnitude. This is due to the combination of rotational cooling, translational cooling, and physical compression that takes place within the cold cell. References 1 and 11 contain detailed descriptions of the dynamics of this system 4 Cold naphthalene is detected using laser induced fluorescence, excited by a pulsed laser (10 Hz, 2 ¹J, 308.0 nm) that is ¯red along the tube axis. The laser 1 0 can be tuned across the strong 80 transition or the weaker 80 origin transition of neutral naphthalene. Fluorescence is collected through quartz windows by PMTs on each side of the cell. One PMT collects light from the upstream half of the cell, while the other only collects from the downstream half; comparison of these two spectra allows for direct measurements of loss and cooling as the gas mixture passes through the cell. 2 Results observed spectrum 7 6.2 K theory 6 5 4 3 2 downstream fluoresence, a.u. 1 0 −6 −4 −2 0 2 4 6 laser frequency, cm−1 Figure 2: Laser induced fluorescence from cold naphthalene (n ¼ 2 £ 1011 cm¡3;T = 6:2 K). Fluorescence was collected by the photomultiplier observing only the downstream (colder) half of the flow tube. The naphthalene observed here has cooled from 300 K to 6 K with minimal (< factor 5) loss to the cell walls. A curve ¯t to T = 6.2 K is shown. 11 ¡3 A typical LIF spectrum of cold naphthalene (n = 2 £ 10 cm , Tcell = 6:2 K) is shown in ¯gure 2, along with a theoretical curve ¯t to 6.2 K § 1.5 K.

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